A good documentary about Joan Rivers would have been enough. Rivers is worth her own documentary. She's a comedian of historical importance, the most successful female stand-up of all time, with an influential career that has lasted almost 50 years - and she's still funny.

But "Joan Rivers: A Piece of Work" is a better film than that. It's one of the best documentaries ever made about show business, about what it really consists of and what it demands. Along the way, almost incidentally, by showing us a woman in her mid-70s still driven to pursue this life of stress and punishment, it offers insight into the performer personality.

Rivers herself says it best. She describes the showbiz life as one of constant rejection; and over the course of the film, we see that that's true. People who fail in show business get rejected all the time. Successes, like Rivers, get rejected most of the time. We see her, an elderly woman of enormous professional stature, hustling for commercials, hustling for gigs, worrying over her material, worrying over a barren schedule, getting bad reviews, playing lousy rooms and hoping for things that never happen. And this is someone who has it good.

It's a life of constant ego battering, so it's no wonder that some performers either crack up or become self-protective egomaniacs. To her credit, Rivers is refreshingly not crazy. Yes, the plastic surgery is bizarre - I don't know why she went that far, though the pressure on aging performers can be brutal. And her apartment looks like a cross between Versailles and an 1890s brothel. But the woman herself is grounded, self-aware and plugged into the current culture. Her new material is observant and daring, and there's nothing remotely nostalgic about it.

As the documentary begins, Rivers is turning 75, and her career is in a lull. Her previous accomplishments offer her only minimal satisfaction. For her, it's all about the present tense. She works. The self-belief is impressive. It would be easy to think, watching the first minutes of the movie, that she's finished, and that being finished would not be so bad. She's had a good run. She's made her mark. She's got money. Why not just pack it in?

And then slowly, over the course of the year, the career comes back. It's no one thing. Rivers does everything. She tries a play. She writes a book. She goes out of town for a stand-up show and ends up in a shouting match with a heckler. She submits to a celebrity roast on television, which places her in the hot seat, having to smile while lesser comedians make vicious jokes about her face and her age. She goes on "Celebrity Apprentice" and has to worry about Donald Trump. As seen from the inside, none of this is remotely glamorous, all of it is stressful and half of it's demeaning.

The film contains a wonderful long shot: Following a tribute to George Carlin, a number of famous comedians are standing in the greenroom, and from the back we see Bill Maher put his arm around Rivers, in a gesture both inclusive and protective. He's so much bigger than she is, and in a flash you realize this dynamo, this force of nature, this shark hungry for every possible gig on the planet is practically a little old lady.

In that moment, we feel protective of her, too, but mostly we just marvel at the strength and the spirit. Joan Rivers is more than a "piece of work." She's an extraordinary person, and this is a great documentary.

-- Advisory: This film contains strong language.

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